1. Field of the Invention
The present invention relates to a method and apparatus for image forming, particularly to a method and apparatus for image forming capable of effectively eliminating color displacement by recognizing a rotational position of a rotating member to meet with its target position at a predetermined time, a rotation drive unit included in the apparatus for rotating the rotating member, and a detachable process cartridge detachably provided to the apparatus and including the rotating member.
2. Discussion of the Background
Recently, market demands for image forming apparatuses producing color images have been increasing.
The image forming apparatuses include different types of color image forming apparatuses having different structures. One of the color image forming apparatuses includes one drum-shaped image bearing member, and is referred to as a one-drum image forming apparatus. The one-drum image forming apparatus repeats four cycles of image forming operations to produce a full-color image. In one cycle of the image forming operations, the drum-shaped image bearing member bears an electrostatic latent image of a single color on a surface thereof. The electrostatic latent image formed according to image data corresponding to the single color is developed as a toner image, and is transferred onto an image receiving member, such as an intermediate transfer member and a recording medium. After four cycles of operations similar to those as described above are performed, a full-color image can be obtained.
Since the one-drum image forming apparatus includes one image bearing member, the apparatus can achieve reduction in size and costs. On the other hand, the one-drum image forming apparatus needs to perform a series of image forming operations, such as a charging operation, an optical writing operation, a developing operation, a transferring operation and so forth, for four cycles to produce a full color image. With this structure, it is difficult to speed up the image forming operations.
The image forming apparatuses include another color image forming apparatus that has a plurality of image bearing members for respective toners of different colors. This color image forming apparatus is referred to as a tandem image forming apparatus. While the tandem image forming apparatus performs similar image forming operations to those performed by the one-drum image forming apparatus, the structures of both image forming apparatuses are different. The plurality of image bearing members of the tandem image forming apparatus bear respective electrostatic latent images on respective surfaces thereof. The respective electrostatic latent images formed on the surfaces of the plurality of respective image bearing members are developed as respective toner images of different colors, and are sequentially transferred onto an image receiving member to produce a full color image in one cycle. That is, the above-described series of image forming operations are performed in one cycle.
Although it is difficult to reduce the size and cost of the tandem image forming apparatus, a full color image can be produced in one cycle of image forming operations, which speeds up the image forming operations.
Further, according to the demands from the market that a color image forming apparatus has a speed level equivalent to that of a monochrome image forming apparatus, the tandem image forming apparatus draws attentions of the market.
However, since the above-described tandem image forming apparatus sequentially overlays color toner images formed on the plurality of image bearing members onto the image receiving member, the overlaid image may have color displacements. The color displacements occur due to several causes such as an eccentricity of a drive gear provided to the image bearing member, a lack of accuracy of gear molding, variations of a rotation speed caused by a joint that engages the drive gear with the image bearing member, and so forth. The eccentricity of the drive gear of the image bearing member periodically causes variations of a surface travel velocity of the image bearing member, resulting in elongation and shrink of lengths in the respective toner images when the toner images are transferred onto the image receiving member. When the periodic elongation and shrink caused by variations of the surface travel velocity of the image bearing member do not agree with those of the other image bearing members, a color displacement occurs. The color displacement may occur in an area that is formed between each of the respective image bearing members and the image receiving member. The area is referred to as a transfer area where the toner images formed on the respective image bearing members are transferred. If the surface travel velocity of the image receiving member in the transfer area changes because of variations of the surface travel velocities of the image bearing members, the eccentricity of a rotating shaft of the image bearing member may also cause the color displacement. When an image bearing member has an eccentricity in its rotating shaft, the image bearing member may have a slowest surface travel velocity at a portion of the surface that is closest to the eccentric rotating shaft, and may have a fastest surface travel velocity at a portion of the surface that is farthest from the eccentric rotating shaft.
Some techniques have been proposed to prevent the color displacements by periodically changing the surface travel velocity of the image bearing member. The tandem color image forming apparatus having the above-described techniques forms a pattern image on the surface of the image receiving member from the plurality of image bearing members. By reading the pattern image, the tandem color image forming apparatus detects periodic variations in the surface travel velocities of the plurality of respective image bearing members. Based on the detection results, the periodic variations in the surface travel velocities of the plurality of the respective image bearing members are adjusted so that the surface travel velocities of the plurality of respective image bearing members can agree with each other on the surface of the image receiving member, and the color displacements can be prevented.
To perform the above-described adjustment, rotational positions of respective image bearing members need to be previously determined. One of the above-described techniques uses a detection mark to detect the rotational positions. The detection mark is a target that moves on a rotation path of the image bearing member and is optically or magnetically detected by a mark detection unit. With the above-described technique, the mark detection unit detects the detection mark every time the detection mark passes the mark detection unit in rotations of the image bearing member, and the rotational position of the image bearing member can be uniquely determined. Therefore, the rotational position of each image bearing member can be determined by detecting the detection mark.
However, when the rotational position of the image bearing member is determined using the technique, a problem occurs as described below.
To determine the rotational position of the image bearing member, the image bearing member is first rotated. After the surface travel velocity of the image bearing member becomes stable, a detecting operation of the detection mark starts. The detection mark is generally detected at a time when a leading end of the detection mark reaches to a detection area of the mark detection unit or at a time when a trailing end of the detection mark passes out the detection area of the mark detection unit. When the mark detection unit is set to detect the detection mark when the leading end of the detection mark goes out of the detection area, if the detecting operation starts immediately after the leading edge of the target goes out of the detection area, the mark detection unit has to wait for another cycle until the leading end of the detection mark comes to the detection area again. That is, the rotational position of the image bearing member cannot be detected until the image bearing member rotates one more cycle, which may delay a start of the above-described adjustments.
As a result of the above-described problem, a start of the image forming operation performed after the above-described adjustments may delay for one rotation of the image bearing member at the maximum, and a first print time after the above-described adjustments may also delay. The above-described series of delay may also occur when the mark detection unit is set to detect the detection mark immediately after the trailing end of the detection mark passed out of the detection area. Since the market strongly demands to reduce the first print time, the reduction of the speed of the first print time is significantly important in the technical field of an image forming apparatus.
As described above, a delay of detecting the detection mark may occur when the rotational position of the image bearing member provided in the image forming apparatus is detected to match the target position at a predetermined time. That is, the above-described problem may also occur when a rotational position of a rotating member is detected to adjust the rotational position of the rotating member to agree with the target position at a predetermined time.